Phytoparasitic nematodes are naturally pathogens, but their interactions with other agents causing diseases make it difficult to measure their real impact on crop yield and to make a large-scale estimate. Phytoparasitic nematodes generally cause yearly losses of between 11 and 14% in economically important crops such as legume, cereal, banana tree, yucca, coconut tree, sugar beet, sugar cane, potato, vegetable, ornamental and fruit crops.
The nematode feeding process can cause a reaction in affected plant cells, resulting in root tip and terminal bud death or weakening, lesion formation and torn tissues, bulges and galls, and stem and leaf wrinkling and deformation. Some of these manifestations are caused by decomposition of the tissue affected by nematode enzymes, which, with or without the aid of toxic metabolites, causes tissue disintegration and cell death. Other symptoms, such as galling caused by nematodes from the genus Meloidogyne, are caused by abnormal cell elongation (hypertrophy), by cell division suppression or by controlled cell division process stimulation resulting in the formation of galls (hyperplasia) or of a large number of lateral roots in or close to infection sites.
In some cases, however, symptoms are brought about by biochemical interactions of the plants with the nematodes, affecting general plant physiology, as well as the role nematodes play in forming wounds through which other pathogens, which are primarily responsible for the damage caused, penetrate said plants.
With the application of the new European Directive for the sustainable use of phytosanitary products, most of the chemicals intended for controlling these plant parasites and pathogens are being withdrawn due to their high toxicity and treatment aggressiveness.
An ecological alternative for treating and controlling phytopathogenic nematodes consists of biologically controlling such nematodes by means of the use of microorganisms with nematicidal activity. In this sense, the use of some bacteria strains belonging to the genera Lysobacter or Stenotrophomonas with nematicidal activity has been described. By way of illustration, various assays have shown that L. enzymogenes strain C3 has nematicidal activity against juvenile and adult forms of various phytopathogenic nematodes and that it causes abnormalities in nematode eggs in culture medium (Chen et al., “Influence of Lysobacter enzymogenes Strain C3 on nematodes”, Journal of Nematology, 38(2):233-239, 2006); specifically, Chen et al., analyzed the influence of L. enzymogenes strain C3 on phytopathogenic nematodes Heterodera schachtii, Meloidogyne javanica, Pratylenchus penetrans and Aphelenchoides fragariae, and observed that (i) H. schachtii egg hatching was 50% in the presence of L. enzymogenes strain C3 in agar; (ii) juvenile forms of M. javanica died 4 days after exposure to a culture broth with L. enzymogenes strain C3 chitin; (iii) immersion of juvenile and adult forms of A. fragariae, M. javanica and P. penetrans in a culture broth of L. enzymogenes strain C3 led to rapid death and disintegration of such nematodes; and (iv) exposure of juvenile forms of H. schachtii to an L. enzymogenes strain C3 culture broth resulted in their rapid immobilization and lysis after 3 days. Nevertheless, Chen et al. do not mention anything about the ability of L. enzymogenes strain C3 to promote plant growth.
Chinese patent application CN 101177671 describes L. enzymogenes strain OH11 and its use in treating plant diseases; although the possibility of using said strain in treating diseases caused by phytopathogenic nematodes is mentioned, nematicidal activity of said strain is not demonstrated.
Ya Li (“University of Nebraska—Lincoln Digital Commons@University of Nebraska—Lincoln Theses, Dissertations, and Student Research in Agronomy and Horticulture Agronomy and Horticulture Department Phenotypic Diversity in Lysobacter enzymogenes in Relations to Biological Control, 2014) discloses the control of plant pathogens by several types of Lysobacter, and in particular by several strains of L. enzymogenes. This document also discusses the ability of L. enzymogenes to promote plant growth, although mentions that no information is available on the ability of L. enzymogenes to stimulate plant growth through mechanisms not related to biologic control of plant pathogens.
Therefore, there is a need for new ecological alternatives for biologically controlling phytopathogenic nematodes.